Methods and apparatus for monitoring and controlling a tool in a borehole

ABSTRACT

The particular embodiment described herein as illustrative of the invention shows methods and apparatus for use with a core slicing tool in a borehole. Various circuits for monitoring and controlling the core slicing operation are disclosed, as well as circuits for deriving directional and depth control information. Means are disclosed for transmitting information from a plurality of information sources in the tool on one conductor pair. Also, automatic bias control means at the surface of the earth are shown for accurately detecting the transmitted information. Power can be selectively applied to either the core slicing circuits or the directional and depth circuits on the same conductor set by providing two different power modes. Means in the tool are responsive to one or the other power mode to switch various circuits in the tool onto various cable conductors. Also, a plurality of core slicing operations are controlled from the surface of the earth on one conductor pair by utilizing switching means responsive to a plurality of current logic states. Additionally, a method of determining the overload operating point of a motor in a tool in a borehole is disclosed.

United States Patent [72] Inventor Herbert J. Hart Houston, Tex. 21 1 Appl. No. 857,252 [22] Filed July 15,1969

Continuation-impart of Ser. No. 802,708, Dec. 31. I968, abandoned. [4S] Patented Aug. 3, I971 73] Assignee Schlumberger Technology Corporation New York, N.Y.

[S4] METHODS AND APPARATUS FOR MONITORING AND CONTROLLING A TOOL IN A BOREHOLE 30 Claims, 23 Drawing Figs.

[52] 0.5. CI 73/153, 73/116, 3 I 8/434, 318/490 [5 I] Int. Cl E21b 49/06 [50] Field of Search 73/116, 151, 153; 318/433,434, 490; 166/250; 175/26, 104

[56] References Cited UNITED STATES PATENTS 3,039,543 6/1962 Loocke 175/26 2,509,743 5/1950 Olinger..... 73/116 2,637,204 5/1953 Short 73/116 3,048,755 8/1962 Leonhard 318/176 X 3,068,350 12/1962 Archer 219/110 Primary E.\'aminer.lerry W. Myracle Atr0rneys- Ernest R. Archambeau, Jr., William J. Beard,

Stewart F. Moore, David L. Moseley, Edward M. Roney and William R. Sherman ABSTRACT: The particular embodiment described herein as illustrative of the invention shows methods and apparatus for use with a core slicing tool in a borehole. Various circuits for monitoring and controlling the core slicing operation are disclosed, as well as circuits for deriving directional and depth control information. Means are disclosed for transmitting information from a plurality of information sources in the tool on one conductor pair. Also, automatic bias control means at the surface of the earth are shown for accurately detecting the transmitted information. Power can be selectively applied to either the core slicing circuits or the directional and depth cir cuits on the same conductor set by providing two different power modes. Means in the tool are responsive to one or the other power mode to switch various circuits in the tool onto various cable conductors. Also, a plurality of core slicing operations are controlled from the surface of the earth on one conductor pair by utilizing switching means responsive to a plurality of current logic states. Additionally, a method of determining the overload operating point of a motor in a tool in a borehole is disclosed.

PATENTEDAUG 3:91! 3,596,511

SHEET 2 UF 8 u U L /37 I H711, la 1r ANCHOR PUMP Herder? J Hal 2 ATTORNEY PMENWB mus 3mm SHEET 5 [IF 8 ATTORNEY METHODS AND APPARATUS FOR MONITORING AND CONTROLLING A TOOL llN A BOREHOlLlE This application is a continuation-in-part of application Ser. No. 802,708, filed Dec. 31, 1968 now abandoned, a division of Pat. No. 3,525,406, filed June 29, 1967.

The present invention pertains to electrical methods and apparatus for monitoring and controlling a tool in a borehole and more particularly, it relates to the monitoring and control of sample taking apparatus for obtaining continuous samples of earth formations along a substantially interval of the wall of a previously drilled borehole.

Heretofore, formation samples have usually been obtained from previously drilled boreholes by explosively propelling into the adjacent wall of a borehole one or more tubular bodies having appropriately arranged forward cutting edges. As these bullets penetrate the borehole wall, a generally cylindrical core of the fonnation material is driven into each bullet so that, when the bullets are subsequently retrieved, the cores in each will be recovered at the surface of the earth for examination. Although such core taking bullets have been highly successful, it would be desirable to obtain a continuous sample of an earth formation along a substantially vertical interval of a borehole.

One way of obtaining such continuous samples of earth formations is shown in US. Pat. No. 3,430,7l3 granted to C. P. Lanmon on Mar. 4, 1969. The aforementioned Lanmon patent shows motor driven cutting wheels which are adapted to cut into the adjoining earth formations so as to slice a sample therefrom. There is also shown a suitable hydraulic pump operated by an electric motor for feeding the cutting wheels into and longitudinally upward through the formation, as well as reversing the direction of feed of the cutting wheels. in addition to this, the aforementioned Lanmon patent includes a suitable electrical motor driven hydraulic pump anchor means for pushing the forward face of the sample taking tool against the borehole wall.

It can be appreciated that when such apparatus for performing the above-mentioned functions is in a tool which is supported in a borehole by a cable, certain electrical monitoring and control apparatus may be necessary. Among other things, it would be desirable to have suitable apparatus located in the downhole tool to ascertain the position of the downhole tool relative to the formation so that the core sample may be taken at a desired depth. it would also be desirable to know the direction of the forward face of the sample takingapparatus as well as the spatial orientation of the tool. in connection with determining the depth of the tool, the length of cable lowered into the borehole will not always give an accurate determination of absolute depth due to such factors as cable stretch. This cable stretch cannot usually be accurately determined since cable stretch is a function of such things as temperature, mud viscosity, etc. One common way of determining the proper relative depth of the downhole tool is to provide logging apparatus in the tool itself and correlate these logs with previously run logs. By so doing, the core samples can be obtained at the desired formation in the borehole.

Additionally, the motor which drives the cutting wheels must be monitored to prevent stalling of the motor caused by an excessive load, e.g., the friction on the cutting wheels becoming excessive. When it becomes apparent that the motor may stall out, it then becomes necessary to take suitable action, as by reversing the feed of the cutting wheels. One reason for this is that, should the cutting motor stall out, it may be difficult, if not impossible, to restart the motor with the cutting wheels stuck since the available starting torque is usually less than the running torque in an electrical motor. In connection with starting the cutting motor, it would be desirable to switch auxiliary capacitance into the motor windings during the starting phase to provide more starting torque.

In the past, it has been difficult to determine just what the stall out point of a motor is in a tool thousands of feet in the borehole due to, for example, the change in cable impedance. Even if it is known at what point the cutting wheel motor will stall, it is difficult to accurately determine when the loadon the cutting wheels is such that the motor is approaching the stall out point. It would be desirable, in connection with this possible stalling of the cutting wheel motor, to take the necessary corrective action rapidly and automatically to keep the motor from stalling. in this connection, since the human reaction time is not always fast enough, especially where, as in this case, the operator is bust with other duties, it would be desirable to take such corrective action automatically without requiring the services of the operator.

in connection with the core slicing operation, it is desirable to have suitable circuitry for controlling the direction of the feed of the cutting wheels and to control the hydraulic anchor means. It would also be desirable, in connection with the core slicing operation, to monitor the hydraulic pump pressure for the anchor means and increase the pump pressure when necessary. In this connection, it is important to perform this function very quickly since if the core slicing tool should become deanchored, there is a chance that the sudden movement of the tool will cause the cutting wheels to bind in the earth formations that are being cut, this stalling the cutting motor. Again, since the output torque of an electrical motor is usually less while starting the motor, it may becomevery difficult or even impossible to free the cutting wheels in this case. For these reasons, it would be desirable to monitor the hydraulic anchor pressure so that rapid and automatic corrective action can be taken in case the pressure becomes too low.

It can be seen from the foregoing monitoring and control functions of the downhole tool, there are very many of such functions which must be performed to perform the core slicing operation in the most desirable manner. Since cables which are available at reasonable cost in the well servicing fieldhave a relatively small number of conductors and since cables having a large number of conductors are bulky, expensive, and generally undesirable, it would be desirable to carry out all of the above-mentioned functions on multiconductor cables which are commercially available.

it is an object of the invention therefore to provide new and improved methods and apparatus for monitoring and controlling the operation ofa tool in a borehole.

it is another object of the present invention to provide new and improved methods and apparatus for monitoring and controlling the operation of a tool for cutting samples of the earth formation traversed by a borehole.

it is still another object of the present invention to provide new and improved methods for determining the overload operating point of a motor in a tool which is lowered into a borehole.

in accordance with one feature of the present invention, methods and apparatus for monitoring and controlling the operation ofa tool in a borehole includes supplying power to a motor in a borehole, measuring an operating characteristic of the motor, and regulating the load on the motor in response to the measured operating characteristic. Another feature of the present invention includes monitoring the pressure of a pressure responsive means, such as an anchor means movable in response to hydraulic pressure, and acting to maintain said pressure above a specified level in response to the monitored pressure.

Another feature of the present invention includes methods and apparatus for selectively supplying power to different circuits in the tool over a set of three conductors. This is accomplished by supplying power in a first mode wherein a potential difference is set up between two conductors of the set to energize a first circuit means in the tool and supplying power in a second mode wherein a substantially equal potential is set up between said two conductors with current return on the third conductor of the set. The information signals can then be supplied to said two conductors for transmission to the surface of the earth. Or, another circuit means can be switched onto another conductor set to be coupled to certain circuits at the surface of the earth in response to one of the power modes.

In accordance with another feature of the invention, a method of determining the overload operating point of a tool in a borehole comprises supplying electrical power to a motor in a tool lowered into a borehole and decreasing the voltage of the power supplied to the motor tosubstantially overload the motor. The method further comprises measuring an operating characteristic of the motor at the overload point, said measured characteristic being representative of the overload operating point when the motor is substantially overloaded.

For a better understanding of the present invention, together with other and further objects thereof, reference is had to the following description taken in connection with the accompanying drawings, the scope of the invention being pointed out in the appended claims.

Referring to the drawings:

FIG. 1 shows a core slicing tool in a borehole and in position to obtain an elongated formation sample;

FIG. 2 is a schematic representation of the intermediate portion of the tool shown in FIG. 1;

FIG. 3 is a schematic representation ofa groove system that may be employed with the tool shown in FIGS. 11 and 2;

FIG. 4 is a partial cross-sectional view taken along the lines 4-4 in FIG. 3;

FIGS. 5a and 512 show a schematic representation of the downhole electrical circuitry in a core slicing tool in accordanee with the present invention;

FIGS. 6a and 6b show a schematic representation of the electrical circuitry at the surface of the earth for responding to the information transmitted from the downhole electrical circuitry of FIG. 5 and controlling portions of the FIG. 5 circuitry;

FIGS. 7a-7f represent voltage waveforms at selected points in the FIG. 5 circuitry;

FIG. 8 graphically represents the relationship of torque versus revolutions per second ofa motor in the downhole tool for various cable lengths;

FIG. 9 shows a circuit that could be substituted for a portion of the FIG. 6 circuitry;

FIGS. MIA-10C show voltage waveforms at various points in the FIG. 9 circuit;

FIG. Ill shows how FIG. 5a fits with 5b and FIG. 6a with 61);

FIGS. 12 and 13 show circuits which could be used with the circuitry of FIG. 6a; and

FIG. M shows a plot of load on the cutting wheels motor versus frequency of the power supplied to the means which feeds the cutting wheels into the formation for use in explaining the operation of the FIG. 13 circuit.

Turning now to FIG. 1, a core slicing tool 10 arranged for use with the present invention is shown suspended from an armored multiconductor cable 11 in a borehole 12 and in position to obtain an elongated prismatic or wedge-shaped sample 13 from the adjacent wall of an earth formation 14. The lower portion of the cable 11 is covered with a suitable insulation material 11a. Located at a desired position on this insulated portion of the cable 11 is an electrode, designated SP, for measuring the naturally occurring or spontaneous formation potential. This SP electrode is connected to a suitable conductor in the cable 12 to provide an indication of this naturally occurring potential at the surface of the earth. U.S. Pat. Nos. 3,430,713 by C. P. Lanmon, 3,430,716 by H. J. Urbanosky, and 3,430,714 by E. F. Brieger granted simultaneously on Mar. 4, 1969 more fully describe those portions of the tool 10 that are not a part of the present invention.

In general the upper housing 15 of the tool 10 preferably encloses typical circuitry for locating the tool at a desired position in the borehole 12 as well as the circuitry of the present invention for controlling the various components in the tool and transmitting information and power through the various conductors in the suspension cable 11. The next lower housing 16 preferably includes a hydraulically actuated wall-engaging anchor means 17 that is selectively extendible against one side of the borehole 12 to shift the forward face of the core slicing tool 10 in the opposite direction and thus anchor the tool 10.

The intermediate housing 18 of the tool 10 supports a pair of similar cutting wheels 19 that are respectively mounted in converging vertical planes and arranged to rotate about independent, outwardly diverging axes themselves lying generally in the same horizontal plane and intersecting each other at a suitable angle. The cutting wheels 19 are suitably arranged and sized in relation to one another so that their peripheral edges will all but come together at about the point of intersection of the three aforementioned planes. Thus, by moving the wheels 19 in unison in a generally vertical direction, the generally wedge-shaped or triangular prismatic sample 13 will be cut from the adjacent formation 14.

To gain entrance for the cutting wheels 19 into the formation 14, the wheels are advanced outwardly and upwardly through a longitudinal opening 20 in the housing and to their outermost lateral position. Then, after a longitudinal cut of a predetermined length has been made, the cutting wheels 19 are returned along an upwardly inclined path and back through the housing opening 20 until they are fully retracted. The cutting wheels 19 then return to their original starting position while still fully retracted.

Motion-translating means (not shown) are arranged in the housing 21 coupled immediately below the intermediate tool housing 18 and operated by the travel of the cutting wheels 18 to rotate a sample receiver in the housing 22 therebelow into position to receive successive formation samples, as at 13, as they are freed by the cutting wheels and to segregate these samples from one another. Since a complete description of the motion-translating means and sample receiver is not essential to a full understanding of the present invention, suffice it to say that the sample receiver is basically comprised of a plurality of upright tubes (not shown) that are successively rotated (by the motion-translating means) about the longitudinal axis of the housing 22 into position to receive one of the formation samples 13. The upper ends of these sample-receiving tubes are, of course, open and their lower ends are closed. For further details, a complete description of the motion-translating means and the sample receiver is found in the aforementioned Urbanosky patent.

Turning now to FIG. 2, a schematic representation is shown of the intermediate housing 18 of the tool If) in which the cutting wheels 19 are confined. In general, the cutting wheels 19 are operatively mounted below an enclosed housing or enclosure 23 that is in turn secured to two parallel tubular members 24 (only one seen in FIG. 2). These tubular members 24 are each slidably disposed about substantially longer, paralleled longitudinal rods 25 (only one seen in FIG. 2) that are secured only at their upper and lower ends to the tool housing 18 and spaced away from the rear wall of the housing. The opposite ends of the tubular members are slidably sealed around the elongated rods 25. A piston member 26 (only one shown in FIG. 2) is fixed at an intermediate position on each of the elongated rods 25 and slidably sealed relative to the internal bore of its associated tubular member 24 to define therein separate upper and lower fluidtight chambers 27 and 28.

Accordingly, it will be appreciated that by developing a higher fluid pressure in the upper hydraulic chambers 27 than that in the lower hydraulic chambers 28, the tubular members 24 and enclosure 28 connected thereto will be moved upwardly along the elongated rods 25 relative to the tool housing 18. Similarly, by imposing a higher pressure in the lower hydraulic chambers 28 than that in the upper hydraulic chambers 27, the enclosure 23 will travel downwardly along the rods 25.

To develop such higher pressures in the chambers 27 and 28, a suitable motor driven hydraulic pump 29 is mounted within the enclosure 28 and connected by fluid lines 30 and 31 to the hydraulic chambers. The pump 29 can be selectively operated from the surface to transfer fluid between the hydraulic chambers to accomplish the desired travel of the enclosure 23 along the elongated rods 25. The hydraulic means for anchoring the tool uses this same type of arrangement (not shown for brevity). To power the cutting wheels 19, a suitably induction motor 32 is also fitted into the enclosure 23 and connected to the cutting wheels by suitable power transmission means 33.

A pair of depending arms 341 (only one shown in FIG. 2) are connected at their lower ends to a right-angle gear drive 35 supporting the cutting wheels 119, with these arms being pivotally connected at their upper ends to the enclosure 23. Outwardly biased guide pins (not shown in FIG. 2) near the free ends of the pivoted arms 3% are slidably disposed in a labyrinthlilte system of grooves 35a (only one system seen in FIG. 2) formed in the interior sidewalls of the intermediate housing 13 on opposite sides of the longitudinal opening 20 therein. These groove systems 35 are so arranged that upward longitudinal travel of the enclosure 23 from its full-line position to its dashed-line position at 36 in FIG. 2 will be effective (through the coaction of the guide pins in their respective groove system) to direct the cutting wheels l9 along the path ABC--D depicted in FIGS. 2 and 3. Then, upon downward travel of the enclosure 23 back to the full-line position shown in FIG. 2, the groove systems 35 and their guides will direct the cutting wheels 19 downward toward their initial position at A.

As seen in FIG. 3, the groove systems 35 are arranged in a closed loop having two parallel longitudinal positions 37 and 38 of unequal length and spaced apart from one another. The shorter grooves 38 are connected at their opposite ends to the longer grooves 37 by oppositely directed inclined grooves 33 and 40 which respectively intersect the longer grooves at longitudinally spaced intermediate points.

Accordingly, as the cutting wheels 19 move along the path A-B, they will be moving upwardly and outwardly as they cut their way into the formation M. Then, as the cutting wheels 19 move upwardly from their position at 3" to their position at C," they will be cutting along a straight path of a length determined by the length of the shorter grooves 38. Upon reaching their position at C," the cutting wheels 19 will be retracted as they move further upwardly and cut their way toward their position at D." Thus, once the cutting wheels 19 have reached the position at D, a prismatic sample 13 with tapered ends will have been cut out of the formation M and will drop into the core receiving housing 22. therebelow.

The groove systems 35 must, of course, be arranged to insure that their respective guide pins are diverted into the lower inclined grooves 39 as the enclosure 23 moves upwardly. Similarly, when the enclosure 23 has reached its uppermost position (as at 36 in FIG. 2), the guide pins must be prevented from reentering the upper grooves 40 so that the cutting wheels W can proceed from their position at D" to their position at lE" and back to their initial position at A.

Accordingly, as seen in FIGS. 3 and 4, an abutment dll is provided in the lower end of each of the longer grooves 37 for preventing the guide pins from entering the longer grooves as they move upwardly. To facilitate the passage of the guide pins, the faces of the abutments 41 are extended along the line of the downwardly facing wall ofthe lower inclined grooves 39 as shown in FIG. 3. Similarly, to insure that the guide pins will not reenter the upper end of the upper inclined grooves 40 as the enclosure 23 is returned downwardly, an abutment 42 (similar to that at 311) is located across the entrance to the upper end of each of the upper inclined grooves d0. Here again, to facilitate the passage of the guide pins, the faces of the abutments d2 are made as a continuation of the right-hand (as viewed in FIG. 3) sidewalls of the longer grooves 37. The height of each abutment, as at 411, is made less than the total depth of its associated groove 37 and inclined ramp or surface, as at 43 (FIG. 3), is provided from the bottom of the groove 37 up to the upper surface of the abutment, with this inclined surface rising in the direction from which the guide pin in that groove is intended to be coming. Thus, as the springbiased guide pins reach their positions they can retract sufficiently to move up the inclined surfaces 433 of the abutments 411 as the enclosure 23 is moved downwardly. Once the guides reach the abrupt faces of the abutments, they will be urged outwardly and return to their normal extended position. The inclined surfaces l l (FIG. 3) on the lower ends of the upper abutments 42 in the slots 38 will, of course, function in the same manner.

Referring now to FIGS. 5a and 5b, there is shown the downhole electrical circuitry of the present invention which has two basic functions. One is to obtain directional information such as the azimuth, relative bearing and inclination of the tool in the borehole as well as running a log of certain characteristics of the adjoining earth formations for depth control purposes. The second function of the downhole circuitry is to monitor and control the core slicing-operation, including the anchoring of the tool in the borehole, and supply certain information to the surface of the earth.

For the directional information and depth control, there must be a sufficient number of conductors to supply the azimuth, relative bearing, and inclination information, as well as the depth control logging information to the surface of the earth. It is also necessary to derive power from the surface of the earth to operate the directional information and depth control equipment in the tool 10. Concerning the functions utilized in connection with the core slicing operation, FIGS. 5a and 31; also includes circuitry for monitoring the rotational speed of the motor 32 driving the cutting wheels 19, circuitry for measuring the position of the cutting wheels 19 along their elongated path as they are cutting the formation material, as well as circuitry for monitoring the hydraulic pressure of the hydraulic anchor means. The FIGS. 5a and 5b apparatus, in connection with the core slicing operation, also includes circuitry for selectively energizing switching means for starting the cutting motor 32, applying hydraulic pressure in the proper chamber (upper or lower) to anchor or release the tool, and applying hydraulic pressure in the proper chamber 2'7 or 23 for moving the cutting wheels up or down.

Before proceeding with the discussion of FIGS. 5a, 5b, 6a and 6b, refer to FIG. 11 to see how they fit together. The letters q through z" correspond to the conductors which pass from FllGS. 5a to 5b and 6a to 612. Hereinafter, FIGS. 50 and 5b will be referred to simply as FIG. 5 and FIGS. 6a and 6b simply as FIG. 6.

Referring now to FIG. 6, the apparatus at the surface of the earth for supplying power to the apparatus utilized in connection with the core slicing operation comprises a variable frequency power source 50 which supplies current through an inhibit gate 51 to the primary winding 53 of a transformer 54. The current developed in the secondary winding 55 of the transformer 54 is supplied between the armor of the cable 11 and through the primary windings 56 and 57 of a transformer 58 to the normally closed contacts of a pair of switches 59:: and 59f of a plurality of ganged switches 59. The common contacts of switches 5% and 59f are connected to the conductors e and j of the multiconductor cable ill. When the switches 59c and 59f are switched to their normally open contacts, the conductors e and f are connected across the secondary winding 60 of a transformer 61. The primary winding 62 of transformer M is connected to a suitable AC power source 63.

It can be seen that when the switches 59:: and 59f are in their normally closed positions, the variable frequency power source 50 develops a voltage across the secondary winding 55 of transformer 5 which causes inphase voltages to be set up on conductors e" and f." 0n the other hand, when the switches are in the normally open position, the AC power supply 63 causes out of phase voltages or a potential difference to be set up across the cable conductors e" and f."

Now referring to FIG. 5, the primary winding 64 of a transformer 65 is connected across the cable conductor e" and in the tool. The secondary winding 66 of transformer 65 is connected to the input of a DC power supply 67. Thus, DC power supply 67 will only be energized when conductors e and T have out of phase voltages, i.e., only when switches 59c and 59f (FIG. 6) are in the normally open positions coupling AC power supply 63 to conductors 2" and f." Also connected across the cable conductors e and in series relationship, are a pair of oppositely phased primary windings 68 and 69 of transformers 70a and 70b respectively. The intermediate point between the primary windings 63 and 69 is connected to the armor of the multiconductor cable 11. A pair of identically phased secondary windings 71 and 72 of transformers 70a and 7012 respectively are connected in series and supplied to the input ofa DC power supply 73.

it can be seen that these windings of transformers 70a and 70b are wound or phased (shown by the dots) so that DC power supply will be energized only when inphase voltages are applied to cable conductors e and f' with the current return through armor, i.e., when switches 59c and 59f(FlG. 6) are in the normally closed position coupling power supply 50 to cable conductors e and f. When energized, the power supply 73 provides a DC supply voltage designated B to the core slicing circuits. The ground for power supply 73 is designated PS" in H0. 5. This 8* voltage is shown at various points in the FIG. circuit.

The cable conductor 2 is also connected in series fashion through the secondary windings 74 of a transformer 76 and to the common contact of switch 77a. The cable conductor is also connected through the secondary winding 75 of transformer 76 to the common contact of a switch 77b. Switches 77a and 77b are energized by a relay solenoid 77 which is energized when DC power supply 73 is energized. The normally open contacts of switches 77a and 77b are connected together and to the common contact of a relay operated switch Kla to supply power for various core slicing functions to be described later. The transformer 76 is utilized to couple pulses indicative of various core slicing operations via the primary winding 78 to the cable conductors e" and f for transmission to the surface of the earth. The secondary windings 74 and 75 of transformer 76 are phased in opposite directions as shown by the dots, so that the power supplied therethrough to the various core slicing functions (via switches 72a and 72b) will not induce a voltage in the primary winding 78, but yet a current supplied to the primary winding 78 will induce a series-aiding current in the secondary windings 74 and 75.

Thus, in operation, when switches 59c and 59f are in the normally open position thus providing out of phase voltages across conductors e and T from power supply 63, DC power supply 67 will become energized and supply DC power for the depth control and directional information apparatus in the tool. This out of phase voltage will not energize DC power supply 73 because of the phasing of transformers 70a and 70b and will not induce a voltage in the primary winding 78 of transformer 76 since relay 77 is only energized when DC power supply 73 is energized. On the other hand, when switches 79c and 79f are placed in the normally closed positions, AC power supply 50 supplies an inphase voltage on cable conductors e and f with the armor being the current return. in this mode, only DC power supply 73 is energized and relay solenoid 77 is energized, thus closing switches 77a and 77b. By utilizing two conductors and armor for this dual mode power supply system, the impedance seen by the power supplies at the surface of the earth is relatively low, thus providing efficient operation of the power distribution system, particularly by providing good impedance matching.

Now concerning the downhole circuitry for providing depth control and directional information to the surface of the earth, the depth control desirably takes the form of gamma ray and spontaneous potential measurements. To provide the gamma ray log, a standard gamma ray scin illation detector 79 supplies pulses representative of the measured natural gamma rays to a suitable amplitude discriminator 80 which passes only those pulses having a voltage amplitude above a certain level. The pulse output from discriminator 80 is supplied to a suitable scaler or frequency divider 81, which comprises one or more flip-flops, whose output is supplied to a suitable cable driver 82 which could comprise, for example, a blocking oscillator. The pulse output from cable driver 82 is coupled through a transformer 83, whose secondary winding 84 is connected between the armor of cable 11 and a cable conductor 5 through a capacitor 85. Also tied to cable conductor "g is the SP electrode of FIG. I through a suitable resistor 86. The capacitor 85 acts to block the SP current from shorting out through secondary winding 84 to armor and resistor 86 tends to keep the SP electrode from loading down the other circuitry tied to cable conductor g." Thus, it can be seen that the depth control information, comprising pulses representative of naturally occurring gamma rays from the earth formations and the SP current (which is a varying DC signal) are supplied to the surface of the earth via cable conductor g. The potential reference electrode for the spontaneous potential measuring circuit is the electrode N at the surface of the earth. The gamma ray pulses are referenced to the armor (see H6. 6). Obviously, since the gamma ray detector and SP electrode are not on depth with the core slicing cutting (see FIG. 1), some depth shifting of SP and gamma ray will be necessary.

The directional information apparatus comprises equipment for measuring the azimuth, relative bearing, and inclination of the tool 110 in the borehole. This is represented by the box 87 designated directional information." This apparatus can comprise for example the apparatus shown in US. Pat. No. 2,746,162 granted to M. F. B. Picard on May 22, 1956. The DC power for the directional information apparatus 87 is derived from DC power supply 67. The output signals from the directional information apparatus are in the form of varying DC signals proportional to the desired information. To facilitate the transmission of this directional information to the surface of the earth, a suitable relay solenoid 88 is connected across the cable conductors e and f. Therefore, when the power on cable conductors e and f is in the depth control and directional mode, i.e. (out of phase) to energize DC power supply 67, relay solenoid 88 will become energized. This switches the output conductors from the directional information circuitry 87 (which are connected to the normally open contacts of the switches controlled by relay solenoid 88) to a plurality of cable conductors a," b, c" and d, which are connected to the common contacts thereof. One of the cable conductors a, b, c or d is utilized as a return conductor, the other three cable conductors containing the azimuth, relative bearing and inclination information.

When the depth and directional information is being measured, the ganged switches 59 are in the normally open position. Thus, the gamma ray pulses between conductor g and the armor, are supplied to an amplifier and pulse rate to DC converter 80a via a transformer 80b. A blocking capacitor 81a is placed in series with the primary winding 800 of transformer 80b to keep primary winding 800 from loading down the SP circuit. The DC output signal from pulse rate to DC converter 80a is recorded by a suitable recorder 82a driven by a shaft 82b as a function of tool depth. Recorder 82a also records the SP potential developed across the SP electrode and potential reference electrode N, the normally open contact of switch 59g being connected to the recorder 82a for this purpose. The mechanical damping of recorder 82a (i.e., the galvanometer pen) acts as a low-pass filter to block the high frequency gamma ray pulses. To record the directional information, the normally open contacts of switches 59a, 59b, 59c and 5911 are supplied to a suitable recorder 83a.

Now concerning how the various core slicing operations are controlled from the surface of the earth, there are shown a plurality of relay solenoids K1, K2, K3 and K4 in series relationship connected across the cable conductor 3" and armor. Also in series with these relay solenoids are a pair of bacl -toback Zener diodes 89 and 90 for keeping the relay solenoids Kl-Kd from loading down the SP measure circuit, i.e., the Zener breakdown voltage of Zener diodes 89 and 90 will be greater than the greatest expected spontaneous potential. Relay solenoids Kl and K2 have Zener diodes 91 and 92 respectively shunted therewith and relay solenoids K3, K6 have diodes 93 and 94 respectively shunted therewith. Cons idering cable conductor g" as positive, diodes 91 and 93 are back-biased and diodes 92 and 94 are forward-biased.

The relay contact Klla associated with relay solenoid K1 has its normally closed contact connected to the common contact of a switch 102a which is energized by relay solenoid K2. The normally closed contact of switch K211 is connected to one input of the motor 29a for operating the hydraulic cutting wheel feed pump. (See FIG. 2). The normally open contact of switch K211 is supplied to the other input ofthe motor 29a. The power for the motor 29a is returned through the armor. The normally open contact of switch Klla is connected to the common contact of a switch K312 which is energized by the relay solenoid M3. The normally closed contact of switch K30 is supplied to one input of the motor 95 which controls a hydraulic anchor pump 96 to move the anchor means l7 (FIG. 1). The normally open contact of switch M311 is supplied to the other input of motor 95, the current from the motor 95 being returned through armor.

Now concerning the relay logic associated with relays K1, K2, K3 and Kdl, these relay solenoids are selected such that the current required to energize solenoids K3 and K4 is much greater than that required to energize solenoids K1 and K2. Now, if a positive voltage is applied to cable conductor 3, (positive is considered to be cable conductor ,g" positive with respect to armor), it can be seen that diodes 92 and 94 will short out relay solenoids K2 and K4 On the other hand, if a negative voltage is applied to cable conductor g," diodes 91 and 93 will short out solenoids K1 and K3. Thus, by supplying a small positive current on conductor g, only relay solenoid Kl will be energized. On the other hand when a large positive current is supplied to the relay solenoids Kl-K4, relay solenoids KI and K3 will both be energized. it can be seen that the same type of things will occur when the polarity is reversed, that is a small negative current will energize relay solenoid K2 only and a large negative current will energize both solenoids K2 and K4. The Zener diodes 91 and 92 across relay solenoids KI and K2 serve to protect their respective relay solenoids from pulling too much current during the large current logic states. In addition to these four current logic states, there is another current logic state, namely, zero current.

Now looking at the relay contacts associated with relay solenoids KI-ltd, the zero current logic state causes the motor 29 to operate in the mode to feed the cutting wheels 19 down by pumping hydraulic fluid into the lower chamber 28 of tubular member 24$ (FIG. 2). In the negative low current logic state (K2 energized), the cutting wheels 119 are fed up by reversing the motor 29a direction and feeding hydraulic pressure into upper chamber 27 of tubular member 24. A capacitor 97 connected across the inputs to'motor 290 provides the necessary phase change to cause the reversal in direction of motor 29a. When a positive low current is supplied to the relay solenoids thus energizing relay solenoid lfill, the relay switch Klla switches to the normally open position thus supplying power to the hydraulic anchor motor 95 in a phase which will cause the hydraulic anchor pressure to increase. The anchoring means can be a differential pressure operated piston arrangement like the one discussed earlier to feed the cutting wheels l9 up and down. Thus, when speaking of increasing hydraulic anchor pressure, this means that the wall-engaging anchor member 17 (see FIG. l) is pushed outward to engage the borehole wall, and when decreased it is retracted. When a positive high current is supplied to the relay solenoids, thus energizing relay solenoids K11 and K3, it can be seen that power will be supplied in the opposite phase to motor 95 thus causing hydraulic anchor pressure to decrease. A capacitor 93 connected across the input leads to motor 95 provides the necessary phase reversal.

Now, concerning the power supply circuit to cutting wheel motor 32, referring to FIG. 6, the power for the motor 32 is derived from a variable voltage AC power source 99, desirably 60 cycles, which supplies the power via a transformer 100 to the cable. Each side of the secondary winding 101 of transformer is split into two conductors which are each individually connected to the normally closed contact of switches 59a, 59b, 59c and 59d. The common contacts of these switches are connected to cable conductors a, b," c and d." Referring to FIG. 5, at the tool, these cable conductors a, [1," 0" and d are connected to the common contacts of the switches energized by relay solenoid 88. (During the core slicing operation, relay solenoid 88 is deenergized.) The normally closed contacts of these switches are recombined into two separate conductors 102, corresponding to the two conductors from the transformer 100 (FIG. 6) and supplied to the motor 32. The third input to motor 32 is supplied from one of the conductors 1102 through a running capacitor 103 to provide the polyphase operation during normal running operation. A starting" capacitor 104 in series with a normally open switch 1(411 energized by relay solenoid K4 is shunted with running capacitor to provide more capacitance, and thus more starting torque, for starting the motor. Thus, the negative high current logic state which energized relay solenoid K4, is for starting the cutting wheel motor 32.

Referring to FIG. 6, there is shown the circuitry at the surface of the earth for providing the various current states for the relay solenoids Kl-Kd. A suitable DC voltage source 105a provides current for the various logic states. The high and low current states are provided by a switch 106 switching between either a high value resistor 107 to provide a low current or a low value resistor 108 to provide a high valueof current. To provide the positive and negative polarity states, a suitable polarity reversing switch 109 is provided, whose output contacts are supplied through a pair of switches 110 to cable conductor g and armor. The connection to cable conductor g is supplied through a switch 59g of ganged switches 59 (the normally closed contact thereof). The primary winding 111 of a transformer 112 is inserted in the conductor between the polarity reversing switch 109 and one of the switches 110.

ln FIG. 6, there are shown the various core slicing functions, i.e. feed cutting wheels down, increase anchor pressure," start cutting wheels motor, release anchor, and feed cutting wheels up. Each of these functions are coupled in a desired manner to the switches 106, 109 and 110 for providing, the various current. logic states to the downhole relay solenoids ll(ll(d. The feed cutting wheels down" function requires a zero current level on the cable conductor g and thus the switches 110 are open for this function. For the remainder of the functions, switches 110 are closed. For the increase anchor pressure function which requires a low positive current, the switch 106 is placed in the low current position and polarity reversing switch 109 is placed in the positive polarity position. The remaining functions are accomplished in a similar manner by controlling the switches 106 and 109 as desired to provide the desired current logic states.

it is recognized that various logic combinations of these functions can be utilized to bring about desired results. For example, it is most important that should the hydraulic anchor pressure be decreasing, thus indicating that the tool may begin slipping, the increase anchor pressure" function should override all of the other core slicing operations. Thus, the dotted line 113 is shown to provide override of the feed cutting wheels down function (zero current), from the increase anchor pressure function. (Obviously, switches 110 must be closed to operate any other current logic state). Perhaps the second most important consideration is the possible stalling of the cutting wheel motor 32, again to prevent the cutting wheels from becoming stuck in the formation. Thus, when it is determined that the cutting wheels motor 32 may stall out, the feed cutting wheels down" function is energized, but only if the increase anchor pressure" function is not energized. It can be seen that various logic combinations of the core slicing functions can be utilized, as desired, depending on the relative importance of each function. While these particular surface logic functions were not explicitly shown, it can be appreciated that one of ordinary skill in the art can arrange suitable logic elements, such as relays, to achieve the desired results.

Whenever the downhole relay solenoids Kl-Kd are energized from the surface, it may be undesirable to have the relay contacts of these relay solenoids interrupt a large amount of current to prevent burn out of the relay contacts. To alleviate this problem, whenever there is change in current to the relay solenoids lib-K4 downhole, a voltage is induced in the secondary winding 114 of transformer 112 by this change in current. The secondary winding 114 is connected in parallel to a pair of one-shots 115 and 116 which are responsive to positive and negative voltage excursions respectively. The outputs of one-shots 115 and 116 are connected through an OR gate 117 to the control input of the inhibit gate 51, which momentarily removes power from the cable conductors e and f. The on-time of one-shots 115 and 116 are set such that will be reapplied to cable conductors e and f shortly after the downhole relay contacts have had a chance to switch.

Now concerning the downhole circuitry for monitoring the core slicing operation, first referring to FIG. 2, there is shown a suitable ramp device 118 whose axis is arranged parallel to the elongated rods 25 but whose surface nearest the enclosure 23 becomes progressively closer to the enclosure 23 as the enclosure moves in an upward direction. A suitable plunger 119 is in slidable contact with the ramp device 118 so that as the enclosure 23 moves upward, the plunger 119 will move a suitable wiper arm of a potentiometer 1211 contained within the enclosure 23.

Now referring to FIG. 5, the potentiometer 1211 has a 8* voltage from DC power supply 73 applied across the resistance portion thereof. The wiper arm of potentiometer 120 is connected to a suitable current amplifier 121 which supplied an output current through an inhibit gate 122 to a capacitor 123. The output impedance of amplifier 121 is sufficiently high so that the output current therefrom will be substantially constant for any given input voltage to current amplifier 121, i.e., any given position of the wiper arm of potentiometer 120. The capacitor 123 charges up at a rate proportional to the current from current amplifier 121. Thus, the voltage on capacitor 123 increases until reaching the threshold voltage of a suitable voltage comparator 124 at which time voltage comparator 124 generates an output signal. The reference voltage for voltage comparator 124 is derived through a pair of resistors 125 and 126 from the supply voltage 13*.

The output signal from voltage comparator 124i energizes a gate 127 which is connected across the capacitor 123 through a suitable discharge resistor 128. The output signal from voltage comparator 124 also energizes another gate 129 which connects the center point between the two resistors 125 and 126 through a suitable low value resistor 130 to ground. The gate 129, by pulling a sufficient amount of current from B" through the resistor 125, causes the reference voltage to voltage comparator 124 to drop to a low reference value. Therefore, the voltage on capacitor 123 which is discharging through resistor 128, must drop to the new low reference voltage before the output signal from voltage comparator 124 will drop to zero, thus deenergizing gates 127 and 129. When gate 129 is deenergized, the reference voltage to voltage comparator 124 is changed to its original high reference level. The capacitor 123, then, begins charging up again as in the manner previously discussed.

Thus, it can be seen that the time for capacitor 123 to discharge from the high reference level to the low reference level will always be substantially the same, and thus the ontime of each pulse of the pulse train output from voltage comparator 124 will be the same. However, the time required for capacitor 123 to charge from the low reference level to the high reference level of voltage comparator 12 1 will be proportional to the current output from amplifier 121. Thus, the off-time," and thus the frequency of the pulses from comparator 124, will be proportional to this current and therefore proportional to the relative longitudinal position of the cutting wheels 19.

It would be desirable to monitor an operating characteristic ofthe cutting wheels motor 32 to determine when the motor is overloaded and ready to stall out. In this connection a suitable disc 32a is placed on the shaft of the motor 32 to monitor the speed of the motor 32. This disc 32a has two portions 131 and 132 thereof made of magnetic material. These magnetic portions are relatively small and diametrically opposite one another on the disc 32a. Located in magnetic proximity to the disc 32a is a suitable pickup coil 133 which has a voltage induced therein whenever either one of the magnetic portions 131 or 132 pass the pickup coil 32a due to the change in mag netic flux. The signals derived from the coil 133 are supplied to a suitable one-shot circuit 134, which has a fixed ontime." The negative going output from one-shot 134 is supplied through a suitable differentiator 135 and diode 136 to a pulse generator 137, which could comprise another one-shot. The positive going output from one-shot 134 and the output from pulse generator 137 are supplied through a suitable OR gate 138 to the control input of inhibit gate 122.

In operation, each motor speed pulse from pickup coil 133 is delayed by delay one-shot 13 1 for a given time interval before being transmitted to the surface of the earth. This time interval is the on-time of one-shot 134. After the negative pulse output from one-shot 134 is differentiated by differentiating circuit 135 and rectified by diode 136, a positive pulse, which time-wise corresponds to the trailing edge of the pulse output from one-shot 134, triggers the pulse generator 137 to generate a pulse for transmission to the surface of the earth. The pulses from one-shot 134 and pulse generator 137, which are applied to inhibit gate 122, cause the capacitor 123 to stop charging up during the time interval which includes the ontimes of both one-shot 134 and pulse generator 137. This inhibits any pulses from being generated from voltage comparator 124 while the cutting wheels motor speed pulses are being generated. However, the capacitor 123 is not discharged during this time, and after inhibit gate 122 is deenergized, the capacitor 123 continues to charge up as in the previously discussed manner.

To couple these pulses onto the cable conductors e and f for transmission to the surface, the output pulses from voltage comparator 124 and pulse generator 137 energize a pair of gates 139 and 140 respectively which are connected in push-pull fashion to the primary winding 78 of transformer 76. The center tap of primary winding 73 is connected to the supply voltage B. In this manner, pulses of opposite polarity will be induced into the secondary windings 74 and 75 depending on whether gate 139 or gate 140 is energized.

Now referring to FIGS. 7a7f, there are shown the voltage waveforms of these pulse generating circuits for a better understanding thereof. Looking at FIG. 7a, there is shown the voltage on the capacitor 123 of FIG. 5. At the extreme lefthand side of FIG. 7a, the voltage charging the capacitor 123 has reached the high level reference voltage of voltage comparator 124. When the voltage on capacitor 123 equals this reference voltage, gate circuits 127 and 129 are energized to cause the capacitor to start discharging, as shown in FIG. 7a, and the reference voltage of voltage comparator 124 to drop to the low reference voltage level. Looking at FIG. 7b, there is shown the resulting cutting wheels position pulse (shown as negative) generated from voltage comparator 124. It can be seen, by comparing the time scales on FIGS. 7a and 7b, that the pulse width of the cutting wheels position pulse is equal to the time for capacitor 123 to discharge from the high reference voltage to the low reference voltage.

Now looking at FIG. 70, there is shown a typical waveform of this cutting wheels position pulse at the surface of the earth. The change in shape is due to such things as cable capacitance. It can be seen that there is a large positive overshoot whose time duration is considerable. Now referring to FIG. 7d, there is shown the cutting wheels motor speed pickup signal induced in pickup coil 133. As stated in connection with FIG. 5, this pickup signal causes a blank out pulse (shown in FIG. 7e) to be applied to inhibit gate 122 of the cutting wheels position pulse generating circuit. Referring to 7 FIG. 7f, there is shown the cutting wheels motor speed pulse which is supplied to the cable. It can be seen that the cutting wheels motor speed pulse of FIG. 7f will be generated at a time when there is very little, if any, of the positive overshoot portion of the cutting wheels position pulse on the cable.

The positions of the cutting wheels position pulse and cutting wheels motor speed pickup signals of FIGS. 7b and 7d represent the worst case to be encountered, i.e. when they occur almost simultaneously. It can be seen that if the cutting wheels motor speed pickup signal of FIG. 7d occurs at a later time than shown, the cutting wheels motor speed pulse of FIG. 7f will be even further removed from the overshoot portion of the cutting wheels position pulse on the cable at the surface of the earth (see FIG. 70).

If the cutting wheels position pulse should occur at a later time with respect to the cutting wheels motor speed pickup signal of FIG. 7d than as shown in FIG. 7b, then the blank out pulse applied to inhibit gate 122 will inhibit the cutting wheels position pulse from being generated. Looking at the right half of FIGS. 7a through 7], which represents a later time, it can be seen that the cutting wheels motor speed pickup signal of FIG. 7d occurs before the cutting position pulse has been generated. Looking at FIG. 7a, the voltage on capacitor 123 which has been charging up towards the high reference voltage is held at a constant voltage once the blank out pulse of FIG. 7e to inhibit gate 122 has been generated. This voltage is maintained on capacitor 1123, as shown in FIG. 7a,v until the blank out pulse of FIG. 72 is removed from inhibit gate 122, at which time the voltage on capacitor 1231 continues rising until it reaches the high level reference voltage and another cutting wheels position pulse is generated.

Thus, it can be seen that by using the apparatus of FIG. for generating the cutting wheels position and motor speed pulses, the cutting wheels motor speed pulses can be transmitted to the surface of the earth on the same conductor pair without hindrance from the cutting wheels position pulses, and yet the position pulses will not be materially affected since capacitor 123 is not discharged during the blank out period. If this were not the case, it can be seen that should the cutting wheels position and motor speed pulses being running at the same frequency, there is a chance that none of the position pulses would be transmitted. The danger is also present to a lesser degree if the frequency of the position pulses is a multiple of the frequency of the motor speed pulses.

Referring back to FIG. 5, there is one more monitoring function present in the downhole apparatus. It would be desirable to monitor the hydraulic pressure of the hydraulic anchor means since as discussed earlier, should the tool begin slipping, the cutting wheels bind in the earth formations. To accomplish this monitoring function, a hydraulic pressure switch M1 is located at a suitable point in the hydraulic anchor means to monitor the pressure applied to the anchor member 17, which switch M1 closes upon the hydraulic pressure falling below a given specified level. (This switch is not shown in FIG. I since hydraulic pressure switches are well known in the art.) One side of this switch Idll is tied to B", the other side being connected through suitable current limiting resistor M2 to the input of inhibit gate 122. The value of resistor M2 is much less than the output impedance of current amplifier 121i and thus when the hydraulic switch Illll closes, a relatively large amount of current will be supplied to the capacitor I23. This large current will cause the capacitor 123 to charge up at a faster rate, thus increasing the frequency of the pulses applied from voltage comparator 124i to gate 139 to beyond the normal frequency of the positive indications. Thus, in reality, there are three sources of information coupled to cable conductors e" and f' via transformer 76, i.e. cutting wheels position, cutting wheels motor speed, and hydraulic anchor pressure.

Now, referring to FIG. 6, the pulses transmitted up the cable on cable conductors e" and f are supplied through the switches we and 59f which are in their normally closed position, thus coupling these pulses onto the secondary windings I43 and 1M of the transformer 58. The center point between the secondary windings I43 and I44 is connected to surface ground (the ground symbol designated S"). The secondary windings I43 and M4 are phased such that the positive (motor speed) pulses are supplied in a positive polarity to a signal processing circuit 145, which is responsive to positive pulses and the negative (position) pulses are also supplied in a positive polarity to another signal processing circuit 146 identical in construction to circuit 145.

One problem that exists in detecting opposite polarity pulses transmitted to the surface of the earth over a conductor pair is that the previously discussed overshoots may trigger the opposite polarity pulse channel at the surface of the earth, thus giving an erroneous indication. One manner of solving this problem is to place suitable voltage discriminators in the pulse detection channels in the surface circuitry. However, the amplitude of the desired data pulses can vary due to such things as changing cable length as the tool is moved through the borehole and the harsh downhole environmental conditions, c.g. temperature. This would present the problem that some desired pulses may not be passed by the discriminator, or noise, such as the overshoots, would be passed as data pulses. The novel signal processing circuits and 146 substantially alleviate this problem.

Now concerning these signal processing circuits 145 and M6, refer to the circuit M5. The positive pulses from secondary winding 1M charge up a capacitor 147 through a forward-biased diode M8 to the peak applied voltage. A suitable high impedance input amplifier 149, such as an emitter follower is responsive to this peak voltage to provide a DC signal to the resistance portion of a potentiometer 150, which DC signal is proportional to the peak value of the pulses induced in secondary winding 144. Thus, the circuitry comprising diode M3, capacitor 147, and emitter follower 149 comprises a peak detector, the capacitor 147 charging up quickly through diode M8 to the peak level, but discharging slowly so as to hold the charge thereon. The wiper arm of potentiometer 15b isset at a certain percentage of the peak voltage to provide a reference voltage to a voltage comparator 151. The pulses from secondary winding 144 are applied to the input of voltage comparator 151. The wiper arm of potentiometer 150 is set so that voltage comparator 151 will pass only the motor speed pulses from secondary winding 144 and block any other pulse such as the overshoot pulses. It has been found that a reference voltage of approximately two-thirds of the peak detected voltage will be sufficient.

Thus, it can be seen that the signal processing circuit 145 acts to vary the bias or reference voltage of the pulses passed by voltage comparator 151 in accordance with the variations in the peak voltage of the detected pulses. The output pulses from voltage comparator 151 could be shaped, if desired, including each pulse being given the same pulse width and amplitude, by a one-shot 152. If the pulses from comparator 151 are shaped properly, one-shot 152 could be omitted. The signal processing circuit 146 acts in the identical manner on the cutting wheels position and hydraulic anchor pressure pulses from secondary winding 143. If desired, only one peak detection circuit could be utilized to bias both channels, but separate circuits are felt to be desirable due to factors which may affect the opposite polarity pulses separately. It should be noted at this point that the blank out of the position pulses in the tool (energization of inhibit gate 122) will aid this automatic bias control operation. That is to say, the occurrence of a motor speed pulse while the overshoot from the cutting wheels position pulse is still present might tend to push the peak voltage on capacitor 147 too high. Since the discharge rate from capacitor 147 is slow, this may tend to cause errors. However, because of the downhole blanking operation, this problem does not exist as concerns the motor speed pulses.

The output pulses from one-shot 152 of signal processing circuit 145 are supplied to a pulse rate to DC converter 153, the design thereof which is well known in the art. In like fashion, the detected cutting wheels position pulses are applied to a pulse rate to DC converter 15 3. The DC output voltage from pulse rate to DC converter 154, which is proportional to the frequency of the cutting wheels position pulses received at the surface of the earth, are applied to a suitable voltmeter 155 and a voltage comparator 156, which could also be a Schmitt trigger, if desired. The reference voltage for voltage comparator 156 is derived from the wiper arm of a suitable potentiometer 157 to which is supplied a constant voltage V. The reference voltage applied to voltage comparator 156 is set such that an output signal will be generated from voltage comparator 156 when the high frequency pulses indicating low hydraulic anchor pressure are received, i.e., the higher frequency will cause the DC output from pulse rate to DC converter 154 to increase substantially. Looking at the meter 155, the normal operating portion thereof for the cutting wheels position indication is represented by the clear portion thereof and the hatched line portion represents low anchor pressure.

When the low hydraulic pressure signal indication is received, it becomes necessary to increase the hydraulic anchor pressure. However, since a faster response time than is obtainable with human operators may be required, this function is performed automatically by applying the output signal from voltage comparator 156 to the increase anchor pressure function, which causes a low positive current to be supplied to the relay solenoids KI-K4 (FIG. 5) to increase the hydraulic anchor pressure downhole.

Looking at the motor speed channel, the output from pulse rate to DC converter 153 is supplied to the negative input ofa differential amplifier 158. A suitable frequency to DC converter 1580 is responsive to the frequency of the power supplied to the cutting wheels motor 32 from AC source 99 to generate a DC reference signal proportional to this frequency of the power supplied to the cutting wheels motor 32 from AC source 99 to generate a DC reference signal proportional to this frequency to the positive input of differential amplifier 158. Differential amplifier 158 supplies a signal representative of the difference between these two input signals to another voltage comparator 159, which could be a Schmitt trigger, for example. The comparator 159 derives its reference voltage from the wiper arms of a potentiometer 160 to which is applied the constant voltage V. When the DC output voltage from differential amplifier 158 exceeds this reference voltage, voltage comparator 159 generates an output signal to the feed cutting wheels down function which causes the switches 110 to be opened thus providing zero current downhole. This causes the enclosure 23 (FIG. 2) to be fed downward to allow the torque of the saw motor to build up again. (If an increase anchor pressure" indication has been received, it takes precedence though.)

Before proceeding with the theory of operation of the apparatus of FIGS. 5 and 6, it would be desirable to first explain how the overload operating point or the stall out point of the cutting wheels motor 32 can be determined when the tool is on the end ofa long cable in the borehole. One common way of determining the overload operating point or stall out point of a polyphase induction motor is to measure the slip. The slip" of a polyphase induction motor is considered to be the difference between the synchronous speed (frequency of the applied power) of the motor and the armature or output shaft speed of the motor, i.e., motor speed. If the resistance of the power distribution system is always constant (like from the power company), this determination of slip is a relatively straight forward operation. However, when as in this case, the motor is on the end of a long cable in the borehole, this determination of slip is not so easy due to the change in cable resistance. This change in resistance may be substantial since the tool may be thousands of feet in the borehole.

Referring now to FIG. 8, there is shown a torque versus speed curve of a typical polyphase induction motor. Curves are shown for no cable length, maximum cable length, and some intermediate cable length, such as 10,000 feet. The dotted line curves in FIG. 8 show the effect of decreasing the voltage applied to the cable at the surface of the earth, at maximum cable length. The normal operating point is to the right of the peak of the particular curve. The peak of each of the curves in FIG. 8 is normally considered to be the critical slip point" or stall out point of the motor. That is to say, when the motor speed falls below the critical slip point for a given cable length, the motor is in the process of stalling. It can be seen that this critical slip point is radically different for the different cable lengths. Thus, it can be seen that it would be very difficult if not impossible to determine the critical slip point of the tool when it is operating in the borehole by measuring this slip when the motor was at the surface of the earth. Knowing the cable length would not help very much either, since such things as temperature change the operating conditions, and especially the resistance of the cable, which changes the voltage applied to the motor.

However, it has been found that if the voltage at the surface of the earth is decreased, the critical slip point will vary only slightly for any given cable length. This decrease in voltage is represented by the dotted line curves in FIG. 8. Thus, by decreasing the voltage applied to the motor until it stalls out, the critical slip point" can be determined when the tool is in the borehole.

Now referring to FIGS. 6 and 8 in conjunction, the method of determining the critical slip point or overload operating point of the saw motor is to first lower the tool to the point in the borehole at which it is desired to start the core slicing operation. The motor is started up and operated at a suitable point, as the operating point 0 on the maximum cable length curve of FIG. 8. Next, the voltage of variable voltage AC source 99 is decreased, thus moving this operating point to the left in FIG. 8, until the motor stalls out and the motor slip is noted. (The output signal from differential amplifier 158 is proportional to the synchronous speed (frequency of AC source 99) minus the motor speed and is thus a measure of slip. Thus, slip is noted on a voltmeter 151 which is calibrated in slip, connected to the output of differential amplifier 158).

The point at which the motor stalls out can be quickly determined since the meter needle will decrease slowly as the voltage applied to the cable conductors a-d from variable voltage AC source 99 is reduced. Then, when the motor stalls out, the needle will drop rapidly to zero. It is this point where the needle starts dropping rapidly to zero that is considered to be the critical slip point. After providing a suitable safety factor, the reference voltage (wiper arm ofpotentiometer 160) to voltage comparator 159 is adjusted so that the cutting wheels will be fed down whenever the slip of motor 32 approaches this critical slip point" or stall out point thus decreasing the load on the motor.

Referring now to FIG. 9, there is shown a more desirable and quicker acting circuit for monitoring the slip of the cutting wheels motor and reversing the cutting wheels feed. The circuit of FIG. 9 is substituted for the pulse rate to DC converter 153, frequency to DC converter 158a and differential amplifier 158 (which is redrawn in FIG. 9) of FIG. 6. More particularly, the output pulses from signal processing circuit 145 are frequency divided by a divide by 2 circuit (e.g., flipflop) and the resulting pulses supplied to a pulse period to DC converter 171. Within the converter 171, the pulses from divider circuit 170 energize a gate 172 which acts to discharge a capacitor 173. The capacitor 173 is supplied with current from a constant current source 174. A suitable peak detector circuit 175, like the one in signal processing circuit 145, supplied the peak voltage on capacitor 173 to a suitable smoothing filter 176. The resulting varying DC signal proportional to the peak voltage of capacitor 173 is supplied to the negative input of the differential amplifier 158.

The AC sinusoidal signal from AC source 59 is supplied to a wave shaping circuit ll77 (e.g. Schinitt trigger) to convert the sine waves to square waves. The resulting square waves are supplied to another pulse period to DC converter 1710, which is identical to converter ll7l. The output of converter 171a is supplied to the positive input of the differential amplifier 158, the resulting difference signal being differentiated by a differentiator ms. The output of differentiator i178 and differential amplifier 153 are summed in a summing network 178a and supplied to the voltage comparator 159.

In operation, the pulse period to DC converters 171i and E711: act to provide signals proportional to the inverse of synchronous speed and motor speed to the differential amplifier lti. By so doing, the response time of the slip detecting circuit is substantially improved. With a little thought, it can be seen that utilizing inverse functions rather than direct functions for speed introduces negligible error in the computation. This operation can better be seen in FIGS. ltlaltlc. FIG. Illa shows the pulse output of signal processing circuit. The pulses of FIG. lltlb shows how the divider circuit I70 acts on these pulses. FIG. llllc shows how the capacitor 1'73 charges at a constant rate until gate 172 energized by the pulses of FIG. llllb resets the capacitor. By this means, the peak voltage which capacitor 173 attains will be proportional to the period of the motor speed pulses. The resulting DC signal from filter 1176 is the dotted line in FIG. We. Thus, as the pulse rate of FIG. llla changes, the amplitude level to differential amplifier 158 will quickly change also. Thus, the large time constant required in the pulse rate to DC converter of FIG. 6 is not needed.

To further quicken up the circuit response time, the differentiator 1178 is utilized, which acts to supply a large voltage output whenever the output of differential amplifier begins increasing. In case this change is relatively slow, the output of differential amplifier 1158 can energize comparator 159 directly. These circuits of FIG. 9 have the effect of allowing the cutting wheels motor to be operated more efficiently by allowing the operating point ll" of FIG. 8 to be pushed further upon the curve, as to the point P.

Now concerning the method of operation of the apparatus of FIGS. 5 and 6, the core slicing tool ill) is lowered into the borehole to a given depth level. The ganged switches 59 are switched to the normally open position to supply power to the DC power supply 67 downhole (FIG. 5) so as to energize the depth control and directional information apparatus. After sufficient depth and directional information has been obtained, (Note-Gamma ray and SI can be run separately), and when the tool is in the position desired to begin the core slicing operation, the ganged switches 59 are switched to the normally closed position thus energizing the DC power supply 73 (FIG. 5) downhole and supplying power to cutting wheels motor 32.

Next, a low positive current is supplied to the relay solenoids Mil-ltd downhole so as to increase the hydraulic anchor pressure (i.e. force hydraulic fluid into the proper chamber of the differential pressure piston arrangement) and therefore anchor the tool at a fixed point in the borehole. Next, a large negative current is supplied to the relay solenoids KlI-K ll downhole from the start cutting wheels motor function so as to switch the capacitor 11041 into the motor 32 power circuit to facilitate starting. After the motor 32 has been started, the previously discussed method for determining the critical slip of "the motor is carried out and the desired reference voltage to voltage comparator 1159 set accordingly. The reference voltage applied to voltage comparator 156, to monitor the hydraulic anchor pressure, is presumed to already have been adjusted.

Now everything is ready for the core slicing operation. To start this operation, a small negative current is supplied to the relay solenoids Kl-IM downhole so as to cause the cutting wheels to be fed upward into the earth formations. The cutting wheels will usually cut through the earth formations in a continuous manner via the grooves shown in FIG. 3 and return to the starting position in readiness for another slicing operation. The frequency of AC power supply 50 is adjusted to provide a continuous slice, i.e. the frequency determines the rate of feed. If, however, the formations are extremely variable, the core slicing operation will be a series of oscillating motions up and down within the formation interval until the entire core is sliced. In this case, the cutting wheels will be fed upwardly until voltage comparator I59 is triggered, at which time the cutting wheels will be fed in a downward direction until the slip reduces to a more desirable level, at which time the output signal from voltage comparator ll59 will drop to zero. When this happens, the relay contacts will be returned to their closed position and the cutting wheels will be again fed upward. This process will be repeated over and over again until the entire core is sliced.

If however during this process, a low hydraulic anchor pressure indication should be sensed by voltage comparator 156, the increase anchor pressure function will cause a low positive current to be supplied to the relay solenoids l(ll--K4l (regardless of the slip condition), thus causing hydraulic fluid to be pumped into the proper chamber of differential pressure piston arrangement until the pressure forcing the anchor member 17 (FIG. ll) outward attains a desired value. During this automatic increase anchor pressure operation, the cutting wheels will no longer be fed upward, thus eliminating the possibility that the motor will stall out. This can be seen, in FIG. 5, by noting the relay logic, wherein the relay contact Klla will switch to the normally open position to increase hydraulic anchor pressure thus removing power from the hydraulic cutting wheel feed motor 29a also.

In the above description, the cutting wheels were fed down whenever the motor slip became too high. By so doing, the cutting wheels become disengaged from the formation and the load on the cutting wheels motor is thus reduced to a relatively low level. By reversing the feed of the cutting wheels each time the motor overload condition is approached, the time for obtaining a formation core sample is increased. However, as discussed earlier, the frequency of the power supplied to the cutting wheels motor can be varied to enable a continuous slice of the formation to be obtained. It would, however, be desirable to provide means for automatically and continuously adjusting the frequency of the power supplied to the cutting wheels motor to bring about this result.

To this end, referring to FIG. 11, there is shown one embodiment for monitoring the slip of cutting wheels motor 32 and adjusting the feed rate of the cutting wheels to maintain a substantially constant feed rate. The motor slip signal from amplifier 15% of FIG. 6a is applied via an input resistor to the negative input of an operational amplifier lttll. The input signal to the positive input of amplifier 181 is supplied from a potentiometer 182. The output signal from amplifier 181 is utilized to control the frequency ofa variable frequency oscillator M3. The output signal from oscillator 183 drives the downhole cutting wheels feed motor 29a via a power amplifier 18% and the transformer 54 of FIG. 6a.

The setting of the potentiometer 182 determines what the slip of the cutting wheels motor 32 will be maintained at. The amplifier 181 produces a control signal which is proportional to the difference between the desired load or slip signal from potentiometer W2 and the actual load or slip signal from amplifier 1158 of FIG. 6a. This control signal varies the frequency of oscillator 183 in such a manner as to maintain the actual load condition substantially equal to the desired load condition. By so doing, the feed rate will be adjusted to maintain the wheels motor 32 at a substantially constant load.

As a practical point, the frequency of power supplied to the cutting wheels feed motor 29a should be maintained between given limits. The frequency should be greater than about 20 Hz. so that the transformer 54 can pass it and less than about 60 Hz. so that the proper voltage (and power) can be maintained at the cutting wheels feed motor.

Turning now to FIG. 112, there is shown a circuit for accomplishing this. In FIG. l2, the motor slip signal from amplifier 158 of FIG. 6a is applied to the negative input ofa current amplifier 1186. The output signal from amplifier I86 is applied via a resistor 187 and diode 188 to the input of the variable frequency oscillator 133 which supplies power to the cutting wheels feed motor 29a via the power amplifier I84 and transformer 54.

A pair of resistors 139 and 190 are connected from a positive DC voltage source to the anode and cathode respectively of diode 1188. The anode of diode 188 is also connected via a resistor 191 to the output of a current amplifier I92. The negative input of amplifier 192 is fed from a potentiometer 193 which is energized with positive and negative DC voltages.

In operation, the potentiometer 193 is set to the desired value or range of values of motor slip or load. In the absence of a load on the motor 32, the output signal from amplifier 186 will be zero and the current supplied to oscillator 133 will be a combination of the current through resistors 190, I89 and I91. The setting of potentiometer 193 thus determines the maximum amount of current supplied to oscillator I33. Since the setting of potentiometer 193 also determines the desired motor load point which may cause the maximum current and thus frequency to exceed the maximum allowable level, a pair of Zener diodes 94 and 195 are positioned on the outputs of amplifiers 186 and 192 to thus limit the voltage output from these amplifiers and thus limit the maximum current level to the oscillator 183 and frequency thereof.

As the load on motor 32 increases, the amplifier 186 will draw the current passing through resistor 189 (and resistor 19! depending on the setting of potentiometer 193) thus decreasing the current applied to oscillator 183. By so doing, the frequency is decreased thus causing the load to return to the desired level or range of levels. As the load continues to increase, eventually the diode 188 will turn off and the only current applied to oscillator 183 will be that passing through resistor 190. Thus, the value of resistor 1190 is selected to set the minimum possible frequency.

Turning now to FIG. 13. there is shown a plot of frequency of power applied to the cutting wheels feed motor 290 versus the load on the cutting wheels motor 32. A family of representative curves represent the various possibilities of operating characteristics of the circuit of FIG. 3 as selected by the setting of potentiometer 193. Any given setting of potentiometer 193 will select a transfer characteristic lying within the family of curves exemplified in FIG. 13. The slope of the inclined portions of these curves is determined by the system gain.

As an example of the system operation, assume that the potentiometer 193 is set to select the curve K98. In this event, the frequency will be varied between and 60 Hz. for loads varying from L to L If the load decreases below L the frequency will remain at 60 Hz. If the load increases above L the frequency will remain at 20 Hz. If the load increases so high as to present a possibility of stalling the cutting wheels motor 32, the voltage comparator 159 of FIG. 6a will be energized to feed the cutting wheels down.

The particular setting of potentiometer 193 is determined by the formation conditions. Potentiometer I93 is set so that the desired load is obtained on the cutting wheels. Variations in formation hardness (due to the negative feedback) will then produce only minor variations around the desired load.

While there have been described what are at present considered to be preferred embodiments of this invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the invention, and it is therefore, intended to cover all such changes and modifications as fall within the true spirit and scope of the invention.

lclaim:

1. Apparatus for monitoring and controlling the operation of a tool in a borehole wherein the tool includes a motor, and load adjusting means coupled thereto and operable in response to control signals applied thereto for regulating the load on said motor, the combination comprising:

means for supplying power to the motor;

means for measuring an operating characteristic of the motor and generating a motor characteristic signal representative thereof; and

control means responsive to said motor characteristic signal for producing said control signals for application to said load adjusting means to regulate the load on said motor and prevent its overloading.

2. The apparatus of claim 1 wherein said control means includes means for comparing said motor characteristic signal with a reference signal representative of a desired state for said operating characteristic and generating an output signal representative of the difference therebetween, and means responsive to said output signal for generating said control signals for regulating the load on said motor to maintain said motor operating characteristic substantially equal to said desired state for said operating characteristic.

3. The apparatus of claim 2 wherein said means for comparing includes means for selectively limiting the range of said output signal.

4. The apparatus ofclaim 8 further including cutting means coupled to said motor and adapted for cutting formation material; and said load adjusting means includes motor driven feed means coupled to said cutting means and adapted for feeding said cutting means into a formation; and wherein said control means includes means for adjusting a parameter of said control signal in response to said motor characteristic signal to maintain said measured motor characteristic relatively constant.

5. The apparatus of claim 4 wherein said control means includes means for selectively limiting the range over which said control signal parameter can be varied.

6. The apparatus of claim 4 wherein said control means further includes means for comparing said motor characteristic signal with a reference signal and producing a threshold output signal if one of said signals exceeds the other signal, and means responsive to said threshold output signal for energizing said motor driven feed means to reverse the direction of said cutting means whenever said motor characteristic signal indicates that the load on said motor exceeds a given level.

7. Apparatus for monitoring and controlling the operation of a tool in a borehole wherein the tool includes a motor, and load adjusting means coupled thereto and operable in response to a control signal applied thereto for regulating the load on said motor, the combination comprising:

means for supplying power at a given frequency to the motor;

means for measuring the rotational speed of the motor output shaft and generating a motor speed signal representative thereof;

means for comparing the frequency of said motor signal with the frequency of the power supplied to said motor and producing an output signal representative of the slip of said motor; and

control means responsive to said output slip signal for producing a control signal for application to said load adjusting means to regulate the load on said motor.

8. The apparatus of claim 7 wherein said control means includes means for comparing said output slip signal with a reference signal and producing a difference signal representative of the difference therebetween, and means responsive to said difference signal for producing said control signal to cause said load adjusting means to maintain the slip of said motor at a relatively constant level.

9. The apparatus of claim 7 wherein said control means includes means for selectively maintaining said control signal within selectable limits.

10. The apparatus of claim 7 further including cutting means coupled to said motor and adapted for cutting formation material and said load adjusting means include motor driven feed means coupled to said motor and adapted for feeding said cutting means into a formation in response to said control signal; and wherein said control means includes means ill for adjusting a parameter of said control signal in response to said output slip signal to maintain said output slip signal relatively constant.

11. The apparatus of claim Ml wherein the frequency of said control signal applied to said motor driven feed means determines the rate at which said cutting means is fed into a formation; and said control means includes variable frequency means adapted to produce said control signal, and means responsive to said output slip signal for controlling the frequency of said output signal produced by said variable frequency to thereby control the feed rate of said cutting means.

12. The apparatus of claim illl wherein said control means further includes means for comparing said output slip signal with a reference signal and producing a threshold output signal if one of said signals exceeds the other signal, and means responsive to said threshold output signal for energizing said motor driven feed means to reverse the direction of said cutting means whenever said motor characteristic signal indicates that the load on said motor exceeds a given level.

13. A method of determining the overload operating point ofa motor located in a tool in a borehole comprising:

lowering the tool into a borehole; supplying electrical power to the motor; decreasing the voltage of the electrical power supplied to the motor to substantially overload the motor; and

measuring an operating characteristic of the motor at the overload point, said measured characteristic being representative of the overload operating point of the motor when the motor is substantially overloaded.

M. The method of claim l3 wherein the measured operating characteristic is the slip of the motor.

15. The method of claim 14 wherein the step of measuring an operating characteristic of the motor includes the steps of;

measuring the rotational speed of the motor armature;

measuring the frequency of the power supplied to the motor; subtracting the measured rotational speed from the measured frequency of the power to obtain an indication of the slip of the motor; and the step of decreasing the voltage until the motor is substantially overloaded comprises decreasing the voltage of the power supplied to the motor until the motor stalls out, said overload operating point being the motor slip when the motor stalls out. 16. Apparatus adapted for controlling a well bore tool ineluding an electric motor, and comprising:

means adapted for supplying power to such a motor; means adapted for measuring the slip characteristic of such a motor as an indication of the load thereon; and

means operable in response to measured variations in said slip characteristic and adapted for regulating the load on such a motor to prevent stalling thereof.

17. The apparatus of claim 116 wherein said means adapted for measuring motor slip include:

first means adapted for measuring the rotational speed of such a motor and producing a first signal representative thereof, second means adapted for measuring the frequency of alterhating current power being supplied to such a motor and producing a second signal representative thereof, and

third means operable in response to said first and second signals for generating a third signal representative of said motor slip; and

said load-regulating means include means operable in response to said third signal for decreasing the load on such a motor when said motor slip exceeds a predetermined level.

lltt. The apparatus of claim 116 wherein said means for measuring said operating characteristic include:

means adapted for measuring the rotational speed of such a motor and supplying a series of pulses at a pulse rate proportional to said measured speed,

first means adapted for converting said pulse rate to a first signal proportional to the inverse of said pulse rate,

second means adapted for providing a second signal proportional to the period of power supplied to such a motor, and

third means operable in response to said first and second signals for generating a third signal proportional to said operating characteristic.

19. The apparatus of claim 18 wherein said first means include:

means adapted for dividing said pulses, and

means adapted for converting the divided pulses to a DC signal proportional to the period of said divided pulses.

24). The apparatus of claim 16 further including:

means adapted for differentiating said measured operating characteristic,

said load-regulating means being coupled to said differentiating means and adapted for regulating the load on such a motor in response to the rate of change of said operating characteristic.

211. The apparatus of claim 16 wherein said load-regulating means include:

means adapted for continuously regulating the load on such a motor to maintain said operating characteristic at a relatively constant level. 22. A method for controlling the operation of a well bore tool including an electric motor and load-regulating means coupled to said motor, and comprising the steps of:

supplying power to said motor; measuring the slip characteristic of said motor as an indication of the load thereon and producing control signals representative of the loads imposed on said motor; and

applying said control signals to said load-regulating means for changing the load imposed on said motor in accordance with measured variations in said slip characteristic to prevent stalling of said motor.

23. The method of claim 22 wherein the step of measuring said slip characteristic of said motor includes the steps of:

measuring the rotational speed of said motor;

measuring the frequency of alternating current power supplied to said motor; and

subtracting said measured rotational speed from said measured frequency.

2d. The method of claim 22 wherein said control signals are applied continuously to said load-regulating means for maintaining said slip characteristic at a selected level.

25. A method for controlling a well bore tool including an electric motor and load-adjusting means operatively coupled to said motor, and comprising the steps of:

supplying power to said motor;

measuring slip characteristic of said motor as an indication of loads imposed thereon and generating a first signal representative thereof:

producing a second signal indicative of a desired load level for said motor; and

regulating said load-adjusting means in response to a selected relationship between said first and second signals for maintaining said motor at a selected load within its normal load range.

as. The method of claim 25 wherein the step of producing said second signal includes the steps of:

comparing said first signal with a reference signal representative of a desired state for said slip characteristic and generating an output signal as a function of the difference therebetween; and

generating said second signal in response to said output signal for regulating the load on said motor to maintain said measured slip characteristic substantially equal to said desired state of said slip characteristic.

27. The method of claim 26 wherein the step of comparing includes the step of:

selectively limiting the range of said output signal.

28. The method of claim 25 wherein the step of producing said second signal includes the steps of:

comparing said first signal with a reference signal indicative of a given load level and producing a threshold output signal if one of said signals exceeds the other signal; and 

1. Apparatus for monitoring and controlling the operation of a tool in a borehole wherein the tool includes a motor, and load adjusting means coupled thereto and operable in response to control signals applied thereto for regulating the load on said motor, the combination comprising: means for supplying power to the motor; means for measuring an operating characteristic of the motor and generating a motor characteristic signal representative thereof; and control means responsive to said motor characteristic signal for producing said control signals for application to said load adjusting means to regulate the load on said motor and prevent its overloading.
 2. The apparatus of claim 1 wherein said control means includes means for comparing said motor characteristic signal with a reference signal representative of a desired state for said operating characteristic and generating an output signal representative of the difference therebetween, and means responsive to said output signal for generating said control signals for regulating the load on said motor to maintain said motor operating characteristic substantially equal to said desired state for said operating characteristic.
 3. The apparatus of claim 2 wherein said means for comparing includes means for selectively limiting the range of said output signal.
 4. The apparatus of claim 8 further including cutting means coupled to said motor and adapted for cutting formation material; and said load adjusting means includes motor driven feed means coupled to said cutting means and adapted for feeding said cutting means into a formation; and wherein said control means includes means for adjusting a parameter of said control signal in response to said motor characteristic signal to maintain said measured motor characteristic relatively constant.
 5. The apparatus of claim 4 wherein said control means includes means for selectively limiting the range over which said control signal parameter can be varied.
 6. The apparatus of claim 4 wherein said control means further includes means for comparing said motoR characteristic signal with a reference signal and producing a threshold output signal if one of said signals exceeds the other signal, and means responsive to said threshold output signal for energizing said motor driven feed means to reverse the direction of said cutting means whenever said motor characteristic signal indicates that the load on said motor exceeds a given level.
 7. Apparatus for monitoring and controlling the operation of a tool in a borehole wherein the tool includes a motor, and load adjusting means coupled thereto and operable in response to a control signal applied thereto for regulating the load on said motor, the combination comprising: means for supplying power at a given frequency to the motor; means for measuring the rotational speed of the motor output shaft and generating a motor speed signal representative thereof; means for comparing the frequency of said motor signal with the frequency of the power supplied to said motor and producing an output signal representative of the slip of said motor; and control means responsive to said output slip signal for producing a control signal for application to said load adjusting means to regulate the load on said motor.
 8. The apparatus of claim 7 wherein said control means includes means for comparing said output slip signal with a reference signal and producing a difference signal representative of the difference therebetween, and means responsive to said difference signal for producing said control signal to cause said load adjusting means to maintain the slip of said motor at a relatively constant level.
 9. The apparatus of claim 7 wherein said control means includes means for selectively maintaining said control signal within selectable limits.
 10. The apparatus of claim 7 further including cutting means coupled to said motor and adapted for cutting formation material and said load adjusting means include motor driven feed means coupled to said motor and adapted for feeding said cutting means into a formation in response to said control signal; and wherein said control means includes means for adjusting a parameter of said control signal in response to said output slip signal to maintain said output slip signal relatively constant.
 11. The apparatus of claim 10 wherein the frequency of said control signal applied to said motor driven feed means determines the rate at which said cutting means is fed into a formation; and said control means includes variable frequency means adapted to produce said control signal, and means responsive to said output slip signal for controlling the frequency of said output signal produced by said variable frequency to thereby control the feed rate of said cutting means.
 12. The apparatus of claim 11 wherein said control means further includes means for comparing said output slip signal with a reference signal and producing a threshold output signal if one of said signals exceeds the other signal, and means responsive to said threshold output signal for energizing said motor driven feed means to reverse the direction of said cutting means whenever said motor characteristic signal indicates that the load on said motor exceeds a given level.
 13. A method of determining the overload operating point of a motor located in a tool in a borehole comprising: lowering the tool into a borehole; supplying electrical power to the motor; decreasing the voltage of the electrical power supplied to the motor to substantially overload the motor; and measuring an operating characteristic of the motor at the overload point, said measured characteristic being representative of the overload operating point of the motor when the motor is substantially overloaded.
 14. The method of claim 13 wherein the measured operating characteristic is the slip of the motor.
 15. The method of claim 14 wherein the step of measuring an operating characteristic of the motor includes the steps of; measuring the rotational speed of the Motor armature; measuring the frequency of the power supplied to the motor; subtracting the measured rotational speed from the measured frequency of the power to obtain an indication of the slip of the motor; and the step of decreasing the voltage until the motor is substantially overloaded comprises decreasing the voltage of the power supplied to the motor until the motor stalls out, said overload operating point being the motor slip when the motor stalls out.
 16. Apparatus adapted for controlling a well bore tool including an electric motor, and comprising: means adapted for supplying power to such a motor; means adapted for measuring the slip characteristic of such a motor as an indication of the load thereon; and means operable in response to measured variations in said slip characteristic and adapted for regulating the load on such a motor to prevent stalling thereof.
 17. The apparatus of claim 16 wherein said means adapted for measuring motor slip include: first means adapted for measuring the rotational speed of such a motor and producing a first signal representative thereof, second means adapted for measuring the frequency of alternating current power being supplied to such a motor and producing a second signal representative thereof, and third means operable in response to said first and second signals for generating a third signal representative of said motor slip; and said load-regulating means include means operable in response to said third signal for decreasing the load on such a motor when said motor slip exceeds a predetermined level.
 18. The apparatus of claim 16 wherein said means for measuring said operating characteristic include: means adapted for measuring the rotational speed of such a motor and supplying a series of pulses at a pulse rate proportional to said measured speed, first means adapted for converting said pulse rate to a first signal proportional to the inverse of said pulse rate, second means adapted for providing a second signal proportional to the period of power supplied to such a motor, and third means operable in response to said first and second signals for generating a third signal proportional to said operating characteristic.
 19. The apparatus of claim 18 wherein said first means include: means adapted for dividing said pulses, and means adapted for converting the divided pulses to a DC signal proportional to the period of said divided pulses.
 20. The apparatus of claim 16 further including: means adapted for differentiating said measured operating characteristic, said load-regulating means being coupled to said differentiating means and adapted for regulating the load on such a motor in response to the rate of change of said operating characteristic.
 21. The apparatus of claim 16 wherein said load-regulating means include: means adapted for continuously regulating the load on such a motor to maintain said operating characteristic at a relatively constant level.
 22. A method for controlling the operation of a well bore tool including an electric motor and load-regulating means coupled to said motor, and comprising the steps of: supplying power to said motor; measuring the slip characteristic of said motor as an indication of the load thereon and producing control signals representative of the loads imposed on said motor; and applying said control signals to said load-regulating means for changing the load imposed on said motor in accordance with measured variations in said slip characteristic to prevent stalling of said motor.
 23. The method of claim 22 wherein the step of measuring said slip characteristic of said motor includes the steps of: measuring the rotational speed of said motor; measuring the frequency of alternating current power supplied to said motor; and subtracting said measured rotational speed from said measured frequency.
 24. The method of claim 22 wherein said controL signals are applied continuously to said load-regulating means for maintaining said slip characteristic at a selected level.
 25. A method for controlling a well bore tool including an electric motor and load-adjusting means operatively coupled to said motor, and comprising the steps of: supplying power to said motor; measuring slip characteristic of said motor as an indication of loads imposed thereon and generating a first signal representative thereof: producing a second signal indicative of a desired load level for said motor; and regulating said load-adjusting means in response to a selected relationship between said first and second signals for maintaining said motor at a selected load within its normal load range.
 26. The method of claim 25 wherein the step of producing said second signal includes the steps of: comparing said first signal with a reference signal representative of a desired state for said slip characteristic and generating an output signal as a function of the difference therebetween; and generating said second signal in response to said output signal for regulating the load on said motor to maintain said measured slip characteristic substantially equal to said desired state of said slip characteristic.
 27. The method of claim 26 wherein the step of comparing includes the step of: selectively limiting the range of said output signal.
 28. The method of claim 25 wherein the step of producing said second signal includes the steps of: comparing said first signal with a reference signal indicative of a given load level and producing a threshold output signal if one of said signals exceeds the other signal; and energizing said load-adjusting means in response to said threshold output signal to reduce the load on said motor whenever said first signal indicates that the load on said motor exceeds said given load level.
 29. The method of claim 25 wherein said well bore tool further includes formation-cutting means operatively driven by said motor and said load-adjusting means include feed means coupled to said formation-cutting means and adapted for controlling the direction of feed of said formation-cutting means, and the step of producing said second signal includes the steps of: comparing said first signal with a reference signal indicative of a given load level and producing a threshold output signal if one of said signals exceeds the other signal; and energizing said feed means in response to said threshold output signal to reverse the feed direction of said formation-cutting means whenever said first signal indicates that the load on said motor exceeds said given load level.
 30. A method for controlling a well bore tool including an electric motor and load-adjusting means coupled to said motor, and comprising the steps of: supplying power at a selected frequency to said motor; measuring the rotational speed of the shaft of said motor and generating a first signal having a frequency representative of said rotational speed; comparing the frequency of said first signal with said selected frequency and producing an output signal representative of the slip of said motor; producing a control signal in response to said output signal; and applying said control signal to said load adjusting means to regulate the load imposed on said motor in accordance with variations of the slip of said motor for preventing stalling of said motor. 